1. Field of the Invention
The present invention relates to a tube expanding bullet and a method of expanding a tube. More particularly, the present invention relates to a tube expanding bullet capable of expanding a tube that is pierced through collared fin holes of heat exchanging fins so as to expand the fin holes and integrate the tube with the heat exchanging fins, and a method of expanding a tube with the tube expanding bullet.
2. Description of the Background Art
In heat exchangers of air conditioners, etc., a plurality of heat exchanging fins are integrated with tubes, through which cooling fluid passes, so as to effectively facilitate heat transfer. A plurality of heat exchanging fins, each of which has a plurality of fin holes, are piled to form a heat exchanging body to integrate the heat exchanging fins with the tubes. The tubes are pierced through the fin holes of the piled heat exchanging fins. Next, the tubes are expanded to integrate the heat exchanging fins with the tubes.
Conventionally, a tube expanding bullet 100 (see FIG. 6) is used to expand the heat exchanging tubes. The bullet 100 is formed having a columnar shape, and a maximum outer diameter is equal to an inner diameter of an expanded tube. The bullet 100 is fixed to a front end of a mandrel 102 by a cone-shaped member 106.
A male screw section 104, which is upwardly extended from the cone-shaped member 106, is pierced through the bullet 100. The male screw section 104 is screwed with the mandrel 102 to fix the bullet 100 to the mandrel 102.
A through-hole 108 is formed in the cone-shaped member 106. A front end of a screw driver can be inserted into the through-hole 108 so as to easily detach the bullet 100.
The tube 120 (see FIG. 7) is expanded by the bullet 100 shown in FIG. 6. The tube 120 is pierced through fin holes 116 of heat exchanging fins 110 as shown in FIG. 7. A collar 112, whose lower end is bent to form a flange section 114, is extended from an edge of each fin hole 116.
As shown in FIG. 7, the bullet 100, which has a circular projected section whose maximum outer diameter is "D", is inserted into the tube 120 which has already been pierced through the fin holes 116. By inserting the bullet 100, an inner diameter T of the tube 120 is extended to an expanded inner diameter T', which is equal to the maximum outer diameter D of the bullet 100.
Conventionally, the bullet 100 has the circular projected section whose maximum outer diameter is D to expand the tube 120 and the fin holes 116 and integrate the fins 110 with the tube 120. Inner circumferential faces of the collars 112 can be tightly fitted to an outer circumferential face of the tube 120.
However, in FIG. 7, some fins 110 are moved as shown by dotted lines 110' when the bullet 100 is passed through the tube 120. Accordingly, the moved fin 110' tightly contacts the adjacent fin 110. If some fins are moved and tightly contact adjacent fins, heat exchanging efficiency of the heat exchanger is lowered.
In order to avoid the tight contact of the fins which occurs by sudden expansion of the tube, the inventor of Japanese Patent Gazette No. 7-124670 invented an improved tube expanding bullet which was disclosed in Japanese Patent Gazette No. 7-124670. The improved bullet 200 is shown in FIG. 8.
The bullet 200 shown in FIG. 8 expands the tube 120 in two stages. Accordingly, the bullet 200 has a front part 202, in which a circular projected section having a maximum outer diameter A is formed; and a rear part 204, in which another circular projected section having a maximum outer diameter B is formed. The relationship between the maximum diameters A, B and an inner diameter T of the unexpanded tube is T&lt;A&lt;B.
By employing the bullet 200 shown in FIG. 8, the tube 120 is expanded by at least two stages, so that the force expanding the tube 120 can be dispersed evenly along the tube wall. With this dispersion, unlike the case of expanding the tube 120 with the bullet 100 shown in FIG. 7, the resulting tight contact of the heat exchanging fins 110 can be decreased.
However, even if the tube is expanded by the bullet 200, the tight contact of the heat exchanging fins cannot be perfectly prevented. In the case of using very thin and light heat exchanging fins, the fins are even more apt to become tightly contacted and displaced.
In an embodiment of Japanese Patent Gazette 7-124670, a small expanding force works to an inner face of the collar when the tube is firstly expanded and an outer face of the expanded tube lightly contacts the inner face of the collar. With such a small expanding force, the firstly-expanded tube cannot expand the fin hole, and the collar cannot be integrated with the firstly expanded tube. In the second expanding stage, the firstly-expanded tube is further expanded and finally integrated with the collar. As described hereinabove, the fin whose collar is expanded by the secondly-expanded tube is displaced and therefore tightly contacts the adjacent fin whose collar has not yet been expanded.